Re: [PATCH net-next 08/10] net: lan966x: add PCIe FDMA XDP support

[Mohsin Bashir <mohsin.bashr@gmail.com>]

> +static int lan966x_xdp_pci_run(struct lan966x_port *port, void *data,
> +			       u32 data_len)
> +{
> +	struct bpf_prog *xdp_prog = port->xdp_prog;
> +	struct lan966x *lan966x = port->lan966x;
> +	struct xdp_buff xdp;
> +	u32 act;
> +
> +	xdp_init_buff(&xdp, lan966x->rx.max_mtu, &port->xdp_rxq);
> +
> +	xdp_prepare_buff(&xdp,
> +			 data - XDP_PACKET_HEADROOM,
> +			 IFH_LEN_BYTES + XDP_PACKET_HEADROOM,
> +			 data_len - IFH_LEN_BYTES,
> +			 false);
> +
> +	act = bpf_prog_run_xdp(xdp_prog, &xdp);
> +	switch (act) {
> +	case XDP_PASS:
> +		return FDMA_PASS;
> +	case XDP_TX:
> +		return lan966x_fdma_pci_xmit_xdpf(port, data, data_len) ?
> +		       FDMA_DROP : FDMA_TX;

What if the BPF program modifies packet boundaries (e.g., 
headroom/tailroom adjustment)? After bpf_prog_run_xdp(), xdp.data and 
xdp.data_end may differ from the original data and data_len, but 
lan966x_fdma_pci_xmit_xdpf() is called with the original values. 
Wouldn't any adjustments made by the XDP program be silently lost?

> +	default:
> +		bpf_warn_invalid_xdp_action(port->dev, xdp_prog, act);
> +		fallthrough;
> +	case XDP_ABORTED:
> +		trace_xdp_exception(port->dev, xdp_prog, act);
> +		fallthrough;
> +	case XDP_DROP:
> +		return FDMA_DROP;
> +	}
> +}
> +